Ultrasonic, flow measuring devices are applied often in process and automation technology. They permit easy determination of volume flow and/or mass flow in a pipeline.
Known ultrasonic, flow measuring devices frequently work according to the Doppler principle or according to the travel-time difference principle. In the travel-time difference principle, the different travel times of ultrasonic pulses as a function of flow direction of the liquid are evaluated. For this, ultrasonic pulses are sent at a certain angle to the tube axis both in, as well as also counter to, the flow direction. From the travel-time difference, the flow velocity, and therewith, in the case of known diameter of the pipeline cross section, the volume flow, can be determined.
Ultrasonic waves are produced, respectively received, with the assistance of ultrasonic transducers. Travel time is ascertained in according to U.S. Pat. No. 5,052,230 by means of short ultrasonic pulses.
In the case of inline, ultrasonic, flow measuring devices, the ultrasonic transducers contact the medium, or fluid. In this way, significantly greater sound power can be coupled into the fluid in comparison to clamp-on-systems, and the ultrasonic transducers can be acoustically decoupled from the measuring tube, whereby the ratio of wanted signal (sound transmission into the fluid) to disturbance signal (sound transmission into the surrounding measuring tube) is improved. Moreover, in this way, in spite of sound measuring paths extending inclined to the main flow, a sound in-coupling extending perpendicularly to the medium-contacting wall can be implemented, which makes the actual measurement effect independent of changes of the velocity of sound.
For fluid contacting mounting of the ultrasonic transducers, lateral openings in the measuring tube are required. The ultrasonic transducers are so secured that the sealing of the measuring tube is assured under all operating conditions. In order to maximize the ratio of wanted signal to disturbance signal, usually an arrangement of the ultrasonic transducers is used, in the case of which the ultrasonic transducers lie opposite one another in a direct line of sight. Alternatively thereto, arrangements are known, in the case of which the sound moves from the transmitter to the receiver via multiple reflections on the measuring tube inner wall.
For the ultrasonic, measuring methods usually applied in inline, ultrasonic, flow measuring devices based on the travel-time difference or a phase difference or a frequency difference, the axis, on which the ultrasonic transducers lie opposite one another, must not be arranged perpendicular to the measuring tube axis, in order to achieve the desired measurement effect. If it is desired, furthermore, that the flow be as undisturbed as possible through the ultrasonic, flow measuring device, excluded likewise is an ultrasonic transducer arrangement parallel to the measuring tube axis, because, in this case, the ultrasonic transducers or reflectors introduced into the measuring tube would lie within the flow.
From these limitations, there results for ultrasonic, flow measuring devices an ultrasonic transducer arrangement, which is typically inclined relative to the measuring tube axis, which leads in connection with the desired medium contact to bores extending transversely through the measuring tube, into which bores the ultrasonic transducers are then externally inserted. If the ultrasonic transducers do not protrude into the flow, e.g. in order that flow losses be minimized and for protecting the units against abrasion or damage, then there result between the ultrasonic transducers and the cylindrical surface of the flowed-through measuring tube, subsequently called the measuring tube boundary surface herein, fluid filled, hollow spaces.
Different flow states can exist in these hollow spaces, especially as a function of the Reynolds (Re) number. These flow states are influenced decisively by the interaction between the measuring tube boundary surface and fluid volume in the ultrasonic transducer bore hollow space. In cases, in which there occur in the hollow space velocity components in the direction of the sound measuring path, these components superimpose on the actual measured variable, namely the velocity components of the main flow in the direction of the sound measuring path. In this way, considerable measurement errors can arise, in the order of magnitude of several percent, depending on the ratio of the ultrasonic transducer bore diameter to the measuring tube inner diameter, respectively ultrasonic transducer hollow space length to measuring path total length.
An approach for correcting this measurement error is to determine the current Re-number and therewith to perform a targeted measuring error correction in the course of the signal processing. Described in U.S. Pat. No. 5,987,997 is a method, which cares for such a subsequent correction of the measured value deviations. Therein, it is provided, based on the ratios of velocities, or the differences of velocities, to determine the Re-number of the flowing fluid along at least two mutually differing measuring paths. This solution is, however, only of limited applicable, since, at the latest, for Re <1000 (=laminar flow profile), the velocity ratios no longer change and therewith a unique determining of the Re-number is no longer possible. Also, for Re >3000, the determination is not always unequivocal. Furthermore, it can in the case of this form of measuring error correction come to considerable additional measured value deviations: In the case of disturbed flow, e.g. behind tube bends or valves, there arise flow states with velocity ratios between different measuring paths in the inline, ultrasonic, flow measuring device, which the signal processing interprets as a certain Re-number, such that a corresponding correction factor should apply. The “real” Re-number, formed from the average flow velocity in the measuring cross section, can, however, be significantly different. Thus, the applied correction factor no longer fits the current flow state, so that an additional measurement error arises.
For preventing interaction between measuring tube boundary surface and the fluid in the above described hollow spaces, FIG. 11 of U.S. Pat. No. 3,906,791 shows a measuring tube flush fitting, lattice insert, which should be acoustically transparent, on the basis of appropriate dimensions. Disadvantageous with this solution is the expected acoustic attenuation, or scattering, of the sound as well as the danger of deposition in the lattice meshes in the case of fluids with solids fractions. FIG. 12 of this document shows a synthetic material, cover plate/membrane for the hollow spaces. Associated with this plate, however, is not only a weakening of the wanted signal, but also sound refraction, which is strongly temperature dependent. Also, the bubble free filling of the hollow space between ultrasonic transducer unit and the plate, a requirement, in the case of use with different static pressures, is quite difficult.
Japanese Patent 2003202254proposes a solution involving a kind of perforated partition to close off the described hollow spaces. The hollow space between ultrasonic transducer unit and perforated partition should be so embodied that laterally directed sound waves rapidly die out. Such an apparatus leads, however, due to the reduced sound opening, to a weakening of the wanted signal, could plug in the case of fluids with solids fractions, and leads, in the case of use in liquids, to possible trapping of air, which likewise affects the wanted signal strength disadvantageously.